The invention relates to a semiconductor device for producing electromagnetic radiation in an active layer-shaped semiconductor region, in which the active-region comprises at least one active layer or wire of a first semiconductor material between barrier layers of a second semiconductor material, the radiation recombination efficiency of the first semiconductor material being high with respect to that of the second semiconductor material.
Semiconductor devices for producing electromagnetic radiation are used in different fields. The present invention relates more particularly to semiconductor devices in which the radiation produced is coherent, so-called semiconductor lasers. The wavelength of the emitted radiation may then lie in the visible range of the spectrum, but, for example, also in the infrared or ultraviolet range.
Semiconductor devices having the features mentioned above result from the desire to manufacture semiconductor lasers having a shorter wavelength than the so-called double heterojunction (DH) lasers used most frequently hitherto, having a layer-shaped active region of, for example, gallium arsenide (GaAs) or gallium aluminum arsenide (AlGaAs)with a smaller energy gap lying between two oppositely doped passive coating layers of a material with an energy gap larger than that of the active material, such as gallium aluminum arsenide (AlGaAs7, in which the larger energy gap is due to a higher aluminum content.
The radiation produced by these known semiconductor lasers generally has (in air) a wavelength of 800 to 900 nm. For various reasons, it is desirable to manufacture lasers emitting radiation of a shorter wavelength. Thus, for example, when information is stored in image and sound carriers (VLP, DOR, compact disk), the required quantity of surface area for one bit of information is proportional to the square of the wavelength of the laser radiation. Consequently, when this wavelength is halved, it is possible to quadruple the information density. An additional advantage is that with shorter wavelengths a simple optical system is sufficient.
In semiconductor devices having the features mentioned above, various effects may occur in the layer structure of the active region, depending upon the construction of this layer structure. A first effect that may occur is the so-called "quantum-well effect".
The "quantum well" effect occurs when a very thin layer of a first semiconductor material is enclosed between two layers of a second semiconductor material with a larger energy gap than the first material. Consequently the effective energy gap in the very thin layer of the first material becomes larger and thus the wavelength of the radiation produced becomes shorter. One or more layers of the first semiconductor material may then be situated (in the active region) between the layers of the second semiconductor material. If the layers of the first semiconductor material lie very close to each other, the so-called "zone folding" effect may occur due to the fact that these layers constitute a superlattice structure. The "zone folding" effect occurs due to the superlattice structure and results in the conversion of "indirect" semiconductor material to effective "direct" semiconductor material with respect to the band transitions of charge carriers. Thus, the radiation transition probability of the charge carriers is increased so that a high radiation density can be attained. For a description of the "quantum well" effect, reference is made inter alia to the article of Holonyak et al in I.E.E.E. Journal of Quantum Electronics, Vol. GE 16, 1980, pages 170-184.
For a description of the "zone folding" effect, reference is made, for example, to the article of Osbourn et al in Applied Physics Letters, Vol. 41. (1982), p. 172-174.
Further, so-called "iso-electronic" doping may take place in the active layer, in which in a semiconductor device having the features described above the first semiconductor material is provided in the active region in the form of a wire or a layer, the dimensions of the wire or the layer, viewed in a direction at right angles to the wire or the layer, being at most equal to the thickness of two monomolecular layers of the first semiconductor material.
Such a semiconductor device is described in Dutch Patent Application No. 8301187 corresponding to U.S. Ser. No. 592,318, now abandoned.
In the laser structures described above, an active layer is located between two semiconductor layers or zones with a larger energy gap than the effective energy gap of the active region. Such passive semiconductor layers which serve to enclose the radiation produced to the greatest possible extent within the active layer have in such lasers the opposite conductivity type to the active layer and are moreover provided with electrodes. Via these electrodes, charge carriers are supplied, which in the active layer lead to the desired population inversion and hence to laser radiation.
Especially in those cases in which the active region has a "quantum well" structure or is obtained by iso-electronic doping, problems may arise because the injection of charge carriers takes place in a direction at right angles to the surfaces of active material having a high radiation recombination power. For a simple "quantum well" structure, this problem is indicated in the article "Very narrow graded barrier single quantum well lasers grown by metal-organic chemical vapour deposition" by D. Kasemset et al, published in "Applied Physics letters" 41 (10), Nov. 15, 1982, p. 912-914. The problem is that for the injected charge carriers, which have to cause population inversion in these layers, the trapping possibility is very small, notably when the layer thickness becomes smaller than the average free path length of the charge carriers in the relevant material (of the active layer).
For the configuration with a simple "quantum well" a solution is proposed in the aforementioned article in which the active layer is situated between two enclosing layers having a variation in the energy gap such that the enclosing layers exhibit over a distance of 220 nm a gradual increase of the energy gap. As a result, scattering of the charge carriers occurs and these carriers are trapped via recombination processes in the actual "quantum well". Thus, it is possible to obtain a simple "quantum well" laser having a width of the "quantum well" (in the case of gallium arsenide) of 75 nm.
In semiconductor lasers provided with isoelectronically doped active regions, the above problems are even more serious because here the active layers (or wires) have a thickness of at most two monomolecular layers and these structures have effectively to be considered as the unit-case of a "quantum well" structure.